In recent years, in a flat panel display device, such as a mobile phone or a personal digital assistant (PDA), in order to enhance protection and beauty of a display, a thin sheet-shaped cover glass is arranged on a front surface of a display so as to cover a region wider than an image display area.
Reduction in weight and thickness is required for this kind of flat panel display device, and to meet the requirement, reduction in thickness is also required for a cover glass for display protection.
However, when the thickness of the cover glass is reduced, strength thereof is lowered, and the cover glass itself may break during use or due to drop thereof during carrying. Accordingly, there is a problem in that the primary role of protecting a display device cannot be performed.
For this reason, in the cover glass of the related art, in order to improve scratch resistance, a float glass produced by a float method is chemically strengthened to form a compressive stress layer in the surface thereof, thereby enhancing scratch resistance of the cover glass.
It has been reported that warpage occurs in a float glass after chemical strengthening, causing deterioration of flatness (Patent Documents 1 to 3). The warpage occurs due to the difference in the degree of behavior of chemical strengthening between a glass surface (hereinafter, referred to as a top surface) which is not in contact with molten tin during float forming and a glass surface (hereinafter, referred to as a bottom surface) which is in contact with molten tin.
The warpage of a float glass becomes large with an increase in the degree of behavior of chemical strengthening. Accordingly, in a chemically strengthened float glass having surface compressive stress of 600 MPa or more and a depth of a compressive stress layer of 15 μm or more, which has been developed to response to the requirement of high scratch resistance, the problem of warpage becomes obvious compared to a chemically strengthened float glass of the related art having surface compressive stress (CS) of about 500 MPa and a depth of a compressive stress layer (DOL) of about 10 μm.
Patent Document 1 discloses a glass strengthening method in which, after a SiO2 film is formed on a glass surface, chemical strengthening is performed to adjust the amount of ions which diffuse into glass during chemical strengthening. Patent Documents 2 and 3 disclose a method in which surface compressive stress on a top surface side is set within a specific range, thereby reducing warpage after chemical strengthening.
In the related art, in order to reduce the problem of warpage, a coping method of decreasing strengthening stress by chemical strengthening, or removing a surface heterogeneous layer through grinding treatment or polishing treatment on at least one surface of the glass, and then performing chemical strengthening, has been carried out.